It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The present disclosure of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data on site conventionally use networks of sensors (here below designated as “hydrophones” with regard to the acquisition of data in a marine environment). Arrays of hydrophones are forming channels. Several channels are distributed along cable in order to form linear acoustic antennas normally referred to as “streamers” or “seismic streamers”.
As shown in FIG. 1, the network of seismic streamers 20a to 20e is towed by a seismic vessel 21. The hydrophones are referenced 16 in FIG. 2, which illustrates in detail the block referenced C in FIG. 1 (i.e. a portion of the streamer referenced 20a).
The seismic method is based on analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate omni-directional seismic wave trains. The pressure wave generated by the seismic source passes through the column of water and insonifies the different layers of the sea bed. Part of the seismic waves (i.e. acoustic signals) reflected are then detected by the hydrophones distributed over the length of the seismic streamers. These acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where the processing of the raw data is carried out.
In practice, it is aimed to carry out an analyze of sea bed with a minimum number of passage of the vessel in the concerned area. For that purpose, the number of streamers implemented in the acoustic network is substantially raised and the length of the streamers may vary between 6 and 15 kilometers, for example.
Control of the positions of streamers lies in the implementation of navigation control devices, commonly referred to as “birds” (white squares referenced 10 in FIG. 1). They are installed at regular intervals (every 300 meters for example) along the seismic streamers. The function of those birds is to guide the streamers between themselves. In other words, the birds are used to control the depth as well as the lateral position of the streamers. For this purpose, and as illustrated in FIG. 2, each bird 10 comprises a body 11 equipped with motorized pivoting wings 12 (or more generally means of mechanical moving) making it possible to modify the position of the streamers laterally between them (this is referred to a horizontal driving) and drive the streamers in immersion (this is referred to a vertical driving).
To carry out the localization of the seismic streamers (allowing a precise horizontal driving of the streamers by the birds), acoustic nodes are distributed along the streamers. These acoustic nodes are represented by hatched squares, referenced 14, in FIGS. 1 and 2. As shown in FIG. 1, some acoustic nodes 14 of the network are integrated in a bird 10 (case of FIG. 2), and other are not.
The acoustic nodes 14 use underwater acoustic communication means, hereafter referred to as electro-acoustic transducers, allowing to estimate the distances between acoustic nodes (named here below “inter-node distances”). More specifically, these transducers are transmitters and receivers of acoustic signals, which can be used to estimate an inter-node distance separating two acoustic nodes (acting as sender node and receiver node respectively) situated on two different streamers (which may be adjacent or not) as a function of an acoustic signal propagation duration measured between these two nodes (i.e. a travel time of the acoustic signal from the sender node to the receiver node). From the acoustic network, this thereby forms a mesh of inter-node distances allowing to know precise horizontal steering of all the streamers. Transducer here is understood to mean either a single electro-acoustic device consisting of a transceiver (emitter/receiver) of acoustic signals, or a combination of a sender device (e.g. a pinger) and a receiver device (e.g. a pressure particle sensor (hydrophone) or a motion particle sensor (accelerometer, geophone . . . )). Usually, each acoustic node comprises an electro-acoustic transducer enabling it to behave alternately as a sender node and a receiver node (for the transmission and the reception, respectively, of acoustic signals). In an alternative embodiment, a first set of nodes act only as sender nodes and a second set of nodes act only as receiver nodes. A third set of nodes (each acting alternately as a sender node and a receiver node) can also be used in combination with the first and second sets of nodes.
The inter-node distance dAB between two nodes A and B can be typically estimated on the basis of the following formula: dAB=c·tAB, with: node A acting as a sender node which transmits an acoustic signal S to node B acting as a receiver node (see example in FIG. 1, with acoustic signal S shown as an arrow between nodes referenced A and B); tAB, the propagation duration (travel time) elapsed between the emission instant and reception instant of the acoustic signal transmitted from the sender node A to the receiver node B (assuming that the receiver node and the sender node are synchronized); and c, a “measured” or “estimated” value of sound speed (also referred to as sound velocity) of the acoustic signal.
FIG. 3 illustrates the binning coverage. We consider two successive shots of a seismic source: the first shot is illustrated in the upper part of FIG. 3 and the second shot is illustrated in the lower part of FIG. 3 (i.e. the source and the streamer are towed from the right to the left in this example). At each shot of the seismic source, a step of processing is the assignment of each channel to a bin. Bins represent local areas (e.g. 8 m×8 m) on the Earth's surface which have been probed by some channels during the seismic survey, i.e. which have been hit by some rays coming from the source and whose reflected ray is received by a channel.
When several traces can be assigned to the same bin, then the signal to noise ratio may be improved with a processing called “stacking” of the seismic data. The number of different rays reflected on the same bin is called “coverage”. One of the aims of a seismic survey is to get a uniform coverage of the binning grid. However, different events can affect the coverage of the binning grid, such as a feather angle α on the streamers S1-S4 (towed by a seismic vessel 21 via a head rigging 43) caused by a lateral sea current 41 (as illustrated in FIG. 4), a V-shape of the streamer network caused by the vessel's wash, or more generally the distortion of streamers. The feather angle α is the angle formed by a streamer (e.g. S1) relative to the axis 42 along which the vessel 21 moves.
During seismic surveys, the areas to cover are actually skimmed by lines. If we observe a binning grid, with the coverage of each bin, we can see some gaps between adjacent lines which are mainly due to feather angle effect on the network. When the coverage between adjacent lines of the survey is poor, then additional lines called “infill lines” are required, which is time and cost-consuming.
In the last decade, prospectors have equipped the streamers with instruments which permit to control them laterally. As already discussed above, these instruments are navigation control devices (“birds”) which allow maintaining a lateral distance between streamers, which have the effect of suppressing the V-shape and any individual streamer distortion. Sometimes, these instruments are also used to guarantee a stable V-shape, which is also beneficial for coverage.
Besides, some current models which include meteorological data and satellite observations, added by onboard Acoustic Doppler Current Profiling (ADCP) permit predicting streamer distortion and controlling the navigation control devices (“birds”) as a function of the current prediction information. This allows to minimize “infill lines” and to maximize four dimensional (4D) repeatability. A four dimensional seismic survey is a three dimensional survey over a same area of the Earth's subsurface at selected time.
However, despite the integration of navigation control devices (“birds”, i.e. means of lateral control of streamers), there is still sometimes a had coverage of the seismic area and/or a lack of repeatability, mainly due to the feather angle of streamers which can change during a vintage and from a vintage to another, or between two adjacent lines, creating gaps in the coverage.
Moreover, on some systems, all the lateral control is referred to a reference streamer, also called “master streamer”. In this case, as all adjacent streamers are referred to the master streamer thanks to a local control of the lateral forces, a feather angle of the master streamer tends to create the same feather angle for all the spread (i.e. all the adjacent streamers). For example, in FIG. 4, if S is the Master streamer, the slave streamers S2, S3 and S4 have the same feather angle α as S1.
Another drawback of these systems is that if no global control of the navigation control devices (in order to operate a lateral control of the master streamer) is carried out, the shape and direction of the master streamer vary with the current, inducing a feather angle on the streamers caused in case of lateral sea current. If a global control is carried out by a navigation system (on board of the seismic vessel), this is not an optimal solution to keep a stable network in the following situations:                disconnection or cut of streamers, each streamer being connected to a seismic data acquisition system onboard the vessel;        break on a telemetry line between a navigation control device (“bird”) and the onboard control system of the navigation control devices;        loss of the link between the navigation system and the control system of the navigation control devices.        
It must also be noted that between each line of a seismic survey, the vessel realizes a turn of approximately 3°/mn. The time required to stabilize the streamer network is important and is mainly dependant on the vessel speed because the network is generally free in feather angle. The streamer network distortion due to a turn is close to the streamer network distortion doe to a lateral sea current. Therefore, the different drawbacks of the prior art solutions, described above in the case of a lateral sea current are substantially the same the case of a turn.